Seasonal Plasma T4 Titers in the Hibernating Lizard Cnemidophorus sexlineatus

A total of 242 Cnemidophorus sexlineatus were collected in biweekly samples from east-central Alabama and west-central Georgia during the period May 1978 to September 1979. Blood, collected in the field, was returned to the lab and subjected to a radioimmunoassay for thyroxine (T4). Mean biweekly plasma T4 titers (ranging from 1.0 to 4.2 ng/ml) were plotted against the collection date. Analysis of these data indicated the presence of a significant (P \u3c 0.01) quadratic (curvilinear) response with lower titers during the summer months of activity and higher titers during other months, thus suggesting definite seasonal changes in plasma T4 titers. These seasonal fluctuations could not be definitively explained in terms of changes in temperature, reproductive condition, or fat stores. Rather, these abrupt seasonal changes in plasma T4 titers coincided best with the entry into and emergence from hibernation. Plasma T4 titers of hibernating lizards were found to be significantly higher (P \u3c 0.05) than those of active lizards